Some crazy s**t went down. The end.
Oh, you need more?
Right, let's think. A big part of the beginning of the season was the superhero bounty. A public bounty was placed on the heads of all the major superheroes - the higher profile the hero, the higher the fee. Everyone - from regular villains of the public to major league supervillains - was getting in on it, trying to claim the bountys. It was eventually revealed that Red Skull was behind it.
The Avengers reformed, with the current roster being Spider-Man, Captain America, Iron Man, Thor, Loki, Hawkeye and Mockingbird, I believe.
After a near brush with death, Steve Rogers decided to go on some soul searching, giving up the mantle of Captain America and handing it over to Bucky Barnes, who has taken Rogers' place on The Avengers.
Bkhedr has masterminded probably the biggest storyline of the season, bringing back Thor and Loki (trapped in the body of a mortal woman) from the dead, and tying them into an ancient prophecy regarding Chthon returning to claim Earth after the age of the Aesir had come to an end. Chthon sent his servants - the Ifrit - to Earth, and there was something of a battle of wills between the heroes and the Ifrit in an attempt to demonstrate to the Living Tribunal that mankind still deserved to live on. Eventually, Chthon emerged on Earth, but was vanquished by the Avengers, thanks largely to the sacrifice of Thunderstrike.
This transitioned into the current storyline going on with us, with The Avengers searching for the 9 Norn Stones, in an attempt to restore Asgard and the 9 Worlds. Loki has just revealed to Thor that he has been decieving him once again, having been collecting the Norns all along behind Thor's back, and betraying him to acquire all the stones Thor had gathered. Loki now only has one more stone to get, before he uses them for his own mysterious ends.
The Mandarin has been causing lots of high profile trouble recently, between sending a giant robot with the brain of Adolf Hitler to attack New York and nearly triggering a war with France.
Norman Osborn has shed his Green Goblin persona and picked up a new alter ego: The Iron Patriot. He has taken on the guise of a government-sponsored superhero in an effort to win over the American public, recently battling and apprehending Dr. Doom.
Sabretooth has steadily risen up amongst the ranks of New York's criminal underworld, engaging in a brutal turf war with The Kingpin where I believe he eventually emerged as victor.
There is plenty of other stuff I know I'm not covering, but hopefully that's a start on getting you to grips with the happenings of the season.